Spatial-Temporal Strategy and Fishing Practices Drive Catch Composition in Fad-Associated Tuna Purse Seine Fisheries

Abstract

The widespread use of drifting fish aggregation devices (FADs) in tuna purse seine fisheries since the mid-1990s has raised sustainability concerns due to increased fishing mortality of unwanted species since, such as small-sized tunas and silky sharks (Carcharhinus falciformis, FAL). A better understanding of ways to mitigate the catch unwanted species while increasing the catch of target species is critical for developing ecosystem-based management approaches. Here, we analyzed fishing logbook of 345 FAD-associated fishing sets collected in the Western and Central Pacific Ocean (WCPO) to assess the influence of spatial-temporal strategies and fishing practices on the catch composition. Results demonstrate that technical adjustments can enhance the potential sustainability of tuna purse seine fisheries. Especially, small-sized tuna catches were concentrated in northern and eastern regions (2°N–6°N, 170°E–175°E), while FAL bycatch exhibited a more widespread distribution. Fishing practices factors, such as the duration of the purse seine gear set (optimal range 8–10 min) and the time of the set relative to sunrise; substantially increased tuna catches while minimizing small-sized tuna catches. Seasonal analysis revealed that the period post-closure (October–December) positively impacted tuna catches and reduce small-sized tuna catches. Moreover, FADs submerged structure at depths of 70 m and 80 m stabilized tuna aggregations, while FAL bycatch remained unaffected by FAD depth.

Keywords: tuna purse seine fishery, fish aggregation device (FAD), bycatch, Western and Central Pacific Ocean, fishing operation parameter, environmental variable